Thermoelectric devices have been in existence for several decades. These devices utilize dissimilar conductive materials subjected to a temperature gradient across their elemental leg lengths to create an Electromotive Force, or EMF. This EMF, in the form of a voltage, is proportional to the intrinsic thermoelectric power of the thermoelements employed and the temperature differential between their hot and cold junctions. This voltage causes an electric current to flow when the circuit is connected to an external load and thus, electric power is generated. Alternatively, current may be introduced into the thermoelectric circuit to move heat, absorbing it at one junction, moving it and dissipating it at the other junction.
The efficiency of a thermoelectric device can be expressed in terms of a figure of merit (Z) for the materials forming the device, wherein Z is defined as: EQU Z=S squared divided by p multiplied by K
Where:
Z is expressed in units X1000, PA1 S is the Seebeck Coefficient in microvolts per degree Celsius, PA1 K is the Thermal Conductivity in watt-cm per degree Celsius, PA1 is the Electrical Resistivity in ohms-cm
From this information, one can see that in order to maximize thermoelectric efficiency, the Seebeck Coefficient (S) must be high and the Thermal Conductivity (K) and Electrical Resistivity (p) must be low in the thermoelectric materials utilized. For applications in the range between 0 Celsius and 200 Celsius, the Bismuth-Tellurium, Bismuth-Selenium, Antimony-Tellurium alloy composition appears to have the highest overall Figure of Merit. Typically, semiconducting compositions, including the alloy mentioned above, have relatively large numerators and fairly large denominators whereas typical metals and their alloys have relatively small numerators and fairly small denominators.
One disadvantage of current thermoelectric devices is the high cost of the semiconducting materials which yield the highest conversion efficiencies available. A reduction in a thermoelement's cross sectional area not only reduces material volume, but increases electrical resistance proportionately. A reduction of element leg length reduces material volume and decreases electrical resistance, but it becomes increasingly difficult to maintain a temperature differential as this leg length is decreased to the point where an impracticable heat exchange mechanism is required to remove the heat faster than it is entering the thermoelectric device. This is due to the thermal conduction characteristics of the thermoelement material. Secondly, as leg lengths are further reduced, fabrication of the thermoelements themselves becomes increasingly difficult due to the semiconductor's fragile nature.
U.S. Pat. No. 5,434,744, granted to Fritz on Jul. 18, 1995 discloses a substrated thermoelectric device in which thermoelemental spacing is less than 0.010 inch and thermoelemental thickness is less than 0.050 inch. In addition, an improved device is claimed to have greater than 300 thermoelements and their said thickness is "approximately" 0.020 inch.
The design of the present invention allows for a further reduction in thermoelemental thickness while still maintaining optimum performance.
U.S. Pat. No. 5,108,515, granted to Ohta on Apr. 28, 1992 discloses a Bi,Te,Se,Sb thermoelemental material which is pulverized to a specific particle size and then forming a green molding which is then sintered.
U.S. Pat. No. 5,246,504, also granted to Ohta on Sep. 21, 1993 is nearly identical in what is claimed to U.S. Pat. No. 5,108,515.
U.S. Pat. No. 5,108,788 and U.S. Pat. No. 5,108,789, both granted to Rauch, Sr. on Jan. 5, 1988 discloses a PbTe thermolemental material in which the compound is: melted, chill cast into an ingot, ground to a particle size of less than 60 mesh, cold pressed to 30-70 kpsi, and finally sintered the resultant green body.
U.S. Pat. No. 3,400,452, granted to Emley on Sep. 10, 1968 discloses using hot isostatic pressure (even, compressive pressure in all directions) to provide metallurgical bonding between the thermoelemental material and the walls of a metal tube in which it is housed.
U.S. Pat. No. 3,601,887, granted to Mitchell on Aug. 31, 1971 also discloses the use of hot isostatic pressure to provide bonding between the inner walls of a tube and the thermoelectric material.
U.S. Pat. No. 5,318,743, granted to Tokiai on Jun. 7, 1994 discloses to "presinter" a Bi,Te,Se,Sb thermoelemental material, then mold the presintered powder and sinter the resultant form also using hot isostatic pressing technology. The actual thermoelements are then cut from the sintered bulk.
U.S. Pat. No. 5,103,286, granted to Ohta on Apr. 7, 1992 discloses a simultaneous sintering and bonding of the thermoelements to themselves and to their respective interconnection members with the absence of pressure to create any of the bonding. Sintering, which is the heating of an aggregate of metal particles in order to create agglomeration does not involve simultaneous pressure.
U.S. Pat. No. 3,554,815, granted to Osborn on Jan. 12, 1971 discloses a device consisting of a thin, flexible substrate in which "bands" of dissimilar thermoelectric material are disposed on opposite sides of the substrate and perforations within the substrate contain a metallic filler to electrically connect each thermoelement.
U.S. Pat. No. 4,343,960, granted to Eguchi on Aug. 10, 1982 discloses a device consisting of a perforated dielectric substrate in which each dissimilar thermoelement is plated, in a pattern, to portions of both faces and to the walls of each thru-hole.
U.S. Pat. No. 4,459,428, granted to Chou on Jul. 10, 1984 discloses a "substrateless" in which each dissimilar thermoelement junction is soldered to conductive plates and an electrical series pattern is created by etching away portions of the conductive plates.
U.S. Pat. No. 3,201,504, granted to Stevens on Aug. 17, 1965 discloses a method of molding a thermoelectric couple in which dielectric sleeve members are inserted into a mold containing a conductive bottom member, powdered dissimilar thermoelectric material is added into their respective sleeves, powdered conductor is placed on top of both thermoelements, and pressing and subsequent sintering of the entire assembly yields a solid thermocouple.
U.S. Pat. No. 3,129,117, granted to Harding on Apr. 14, 1964 discloses a method of manufacturing a thermoelectric element utilizing hot pressing in a direction perpendicular to current flow through the thermoelement.
U.S. Pat. No. 3,182,391, granted to Charland on May 11, 1965 dislcoses a process for forming, in one step, a thermoelement with a metallic electrical contact at one end which comprises consolidating the thermoelectric material and metallic contact plate within a die cavity which is then hot pressed and removed from the mold cavity.
Although many prior art documents list hot/cold pressing, isostatic pressing and sintering of thermoelement materials, cast in separate molds, to yield whole thermoelements for the later assembly of complete devices, the present invention is unique in that thermoelement material is hot pressed directly onto their respective interconnection members. This "process in place" concept eliminates separate mold steps in addition to creating excellent bonding between the thermoelement and it's interconnection member.
Lastly, for the thermoelectric device to see larger scale deployment, it must become competitive with the conventional alternatives. Therefore, to be competitive, it is necessary that the overall device fabrication costs be kept to a minimum. Current fabrication costs are high partly due to the device's own design. A more manufacturable friendly design can reduce the amount of processing steps currently required and possibly reduce material requirements resulting in overall size, weight, and cost reductions. In addition, new and more novel methods for the manufacture of the actual thermoelement material and the subsequent fabrication of the thermoelements need to be developed to further reduce costs.